1. Field of the Invention
The present invention relates to an image display apparatus and a method of driving the same.
2. Description of the Related Art
The demand of display apparatuses remarkably increases in recent years. Especially, flat-panel display apparatuses such as a liquid crystal display apparatus (LCD) or a plasma display apparatus (PD) are greatly expected. As one of the flat-panel display apparatuses, a light emitting type image display apparatus such as an electroluminescence (EL) display apparatus has a high visibility and is superior in a view angle. The light emitting type image display apparatus does not need a backlight, unlike the LCD apparatus. Also, attention is paid to an organic electroluminescence image display apparatus as a plane type display apparatus excellent in response characteristics.
As a driving system of a dot matrix type display apparatus using light emitting elements, there are a simple matrix driving system and an active matrix driving system. In the simple matrix driving system, usually, scan electrodes (scan lines) as cathode electrodes and data electrodes (data lines) as transparent anodes electrodes are arranged to intersect each other. The light emitting element is arranged to be put between the scan electrode and the data electrode. In case of the above-mentioned simple matrix driving system, a light emitting time is shortened as the number of scan electrodes increases. As a result, a duty ratio becomes smaller. Therefore, the brightness of the organic EL display apparatus decreases.
In case of a single scan driving system, scan electrodes are sequentially scanned one by one so as to drive light emitting elements. Therefore, when the number of scan electrodes increases, the duty ratio becomes small, so that the brightness of the organic EL display apparatus decreases in inverse proportion to the number of scan electrodes. For example, in case of an organic EL display apparatus of a QVGA class, when the number of scan electrodes is 240 and the duty ratio is 1/240. At this time, the brightness of the display apparatus is about 70 cd/m2. In viewpoint of the brightness, it is difficult to view the display apparatus in a situation other than a dark situation like night.
A driving system called a double scan driving system is known against the single scan driving system. In the double scan driving system, scan electrodes on a row side are sequentially driven two by two to increase the brightness of the display apparatus. For example, in case of a color organic EL display apparatus of the QVGA class, horizontal scan lines are equally grouped into two groups of 120 scan lines. One scan line of each of the two groups is driven for one scan drive time and the duty ratio is 1/120. Such a double scan driving system is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 61-264876). In the double scan driving system, the brightness of the organic EL display apparatus can be improved. However, the brightness which is higher than the brightness achieved in the double scan drive system is sometimes required. Also, the double scan driving system needs the power consumption larger than the single scan driving system.
In case of the organic EL element, the emitted light brightness is proportional to a current density in the light emitting pixel. Therefore, to increase the brightness of the organic EL display apparatus, a method is employed in which the drive voltage of the organic EL element is increased so that the current density of the organic EL element is increased. In this method, however, the increase of the drive voltage shortens a life of the organic EL element. Also, it is necessary to provide a voltage adjusting circuit every scan electrode or data electrode. Therefore, the circuit structure becomes complicated and the control becomes complicated. As a result, the cost of the product increases.
As described above, in case of driving an image display apparatus in accordance with the simple matrix driving system, conventionally, it is necessary to change the drive voltage or a pulse duration to change the brightness and power consumption of the display apparatus. However, the circuit for changing the pulse duration is complicated.
Also, conventionally, as the row scanning method in the simple matrix driving system, only the single scanning system and the double scanning system are provided.
Moreover, in the conventional image display apparatus, only one of the single scanning system and the double scanning system is fixedly adopted.
In conjunction with the above description, an image display apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 61-264876). In this reference, phosphor is applied on a screen to emit light when an electron beam is irradiated. An electron beam source generates an electron beam for each of vertical sections obtained by dividing the screen into a vertical direction. A dividing section divides the electron beam for each of horizontal sections obtained by dividing the screen into a horizontal direction. A polarizing electrode polarizes the electron beam in a plurality of steps in each of the vertical direction and the horizontal direction on the way of a path from the electron beam source to the screen. A beam control electrode controls a current quantity of the electron beam to be irradiated on the screen for every horizontal section, and controls the light quantity from each pixel on the screen. A focusing electrode controls the size of the electron beam on the phosphor screen of each pixel. A back electrode controls a quantity of the electron beam from the electron beam source. An acceleration electrode accelerates the electron beam to the screen. The beam control electrode is equally divided into an upper portion and a lower portion in the vertical direction, and signals shifted by ½ field are applied to the upper and lower portions of the beam control electrode.
Also, a method of setting a frame frequency in a liquid crystal display apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-281463). In this reference, the display apparatus includes a plurality of display elements driven by a plurality of scan lines and a plurality of data lines, respectively. The liquid crystal display apparatus selects h of the scan lines (h is an integer equal to or more than 2) at the same time. A frame frequency is set in accordance with the response speed of liquid crystal material.
Also, a liquid crystal driving circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-269752). In this reference, the liquid crystal driving circuit has an output potential for duty drive as frame time divisional drive and an output potential for static drive as frame drive, and outputs one of the output potentials from the output terminal.
Also, an electric field radiation color display apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-254964). The electric field radiation color display apparatus has a frame memory which stores picture data formatted for a frame sequential display. A switching section switches a mode between a usual power consumption mode and a reduced power consumption mode. When the display apparatus is in the reduced power consumption mode, a section carries out a monochromatic display. In the monochromatic display, the frame memory is bypassed. In addition, in the above reference, a display system is composed of an emitter plate addressable in a matrix and 3-color positive plate. In the reduced power consumption mode, the display apparatus is switched from a color mode to the monochromatic mode. By a power reducing unit, green brightness data for a monochromatic display bypasses the frame memory and is connected from the first multiplexer to the second multiplexer directly through a 3-state buffer. The buffer brings about the separation of a bypass line in case of a color display. Because the frame memory can be bypassed in case of the monochromatic display, the power of 1 W can be reduced from the frame memory in a standby mode.
Also, a display apparatus is disclosed in Japanese Laid Open Patent application (P2000-105573A). In this reference, the display apparatus is composed of a substrate, electrodes on the substrate, and an organic light emitting layer put between the electrodes. A display region is divided into a first display region on which a predetermined data is always displayed, and a second display region on which data corresponding to a specific situation is displayed.
Also, a display apparatus is disclosed in Japanese Laid Open Patent application (P2000-112435A). In this reference, the matrix type display apparatus has a function to activate a region of a display region and to inactivate the other region of the display region. At least one of the position, area size, and display contents of the activated display region can be changed for every predetermined time interval.